1. Field of the Invention
The present invention relates to a method of fabricating polycrystalline silicon for a thin film transistor (TFT), a mask pattern used for the method, and a method of fabricating a flat panel display device using the same.
2. Description of the Related Art
In general, sequential lateral solidification (SLS) is a technique in which an amorphous silicon layer is irradiated with a laser beam two or more times consecutively to cause grains of crystal silicon to grow laterally, thereby crystallizing the amorphous silicon layer. A polycrystalline silicon grain fabricated by the SLS technique (or method) has a cylindrical shape elongated in one direction, and a grain boundary is formed between adjacent grains due to the finite size of the grains.
According to the SLS method, a polycrystalline or single crystalline particle can be used to form a large silicon grain on a substrate. It has been reported that, when a thin film transistor (TFT) is fabricated in this manner, characteristics similar to those of a TFT fabricated with a single crystalline silicon can be obtained.
FIGS. 1A, 1B and 1C illustrate a conventional SLS method.
According to the SLS method, as illustrated in FIG. 1A, a laser beam is radiated onto an amorphous silicon thin film layer through a mask having a region through which the laser beam can pass and a region through which the laser beam cannot pass, and thus amorphous silicon of the thin film layer melts at the region through which the laser beam passes.
When cooling begins after the laser beam irradiation is finished, crystallization first occurs at an interface between the amorphous silicon and the molten silicon. Latent heat of solidification generated during the crystallization forms a temperature gradient in which temperature gradually decreases along a direction from the interface between the amorphous silicon and the molten silicon to the molten silicon layer.
As such, referring to FIG. 1B, heat flows in a direction from a mask interface to a middle of the molten silicon layer. Consequently, polycrystalline silicon grains laterally grow until the molten silicon layer is completely solidified, and thus a polycrystalline silicon thin film layer having cylindrical grains elongated in one direction is formed.
Here, the grains grow from a non-molten amorphous silicon layer, and a grain boundary is formed where grains growing in opposite directions contact each other. Such a grain boundary, formed perpendicular to a direction of grain growth, is referred to as a “primary grain boundary”.
Subsequently, as illustrated in FIG. 1C, the mask is moved incrementally to expose a part of the amorphous silicon thin film layer and a part of the previously crystallized polycrystalline silicon layer, and the laser beam irradiation is repeated. Then, as a result of the amorphous silicon and crystallized silicon being melted and then cooled down, silicon atoms become attached to the previously formed polycrystalline silicon grains that are covered by the mask and thus have not melted, and thus lengths of the grains increase.
Therefore, according to the SLS method, it is possible to control a grain size according to the size of a mask pattern and the number of repetitions of a laser beam irradiation.
FIG. 2 illustrates a conventional mask pattern employed in a polycrystalline silicon thin film fabrication method using a conventional 2-shot SLS method, and FIG. 3 illustrates an energy density of a laser beam used when polycrystalline silicon is fabricated using the mask pattern shown in FIG. 2, and the polycrystalline silicon fabricated thereby.
Referring to FIG. 2, when crystallization is performed using the mask pattern of FIG. 2 and energy density in a laser beam is not substantially uniform (i.e., an energy density at a middle portion is higher than that at an edge portion), as illustrated in FIG. 3, the crystallinity of the crystallized polycrystalline silicon may vary between portions irradiated with the laser beam (or pulse). Such varying crystallinity may affect TFT characteristics.
When the laser energy density is not substantially uniform while amorphous silicon is crystallized according to the SLS method, non-uniformity in a luminance of a display employing the crystallized silicon may result.
FIGS. 4A, 4B, and 4C are plan views of a polycrystalline thin film in different stages of formation according to a crystallization method using a mask pattern employed in a method of fabricating a polycrystalline silicon thin film using a laser shot mixing technique according to the conventional SLS method. According to the laser shot mixing technique, which is performed along an x axis of the mask pattern, i.e., a laser beam is scanned along an x-direction, crystallization regions are formed at transparent patterns along the x axis, the transparent patterns are shifted by a distance (e.g., a predetermined distance) along the direction of a y axis perpendicular to the x axis, and thus non-crystallized regions are crystallized.
Referring first to FIG. 4A, after a laser beam is radiated onto an amorphous silicon layer using a common mask having a transparent region and a non-transparent region, amorphous silicon melts, and then the molten amorphous silicon is solidified to thereby form polycrystalline silicon. Subsequently, as illustrated in FIG. 4B, when the mask is shifted by a distance (e.g. a predetermined distance) and the laser beam is radiated again, polycrystalline silicon of the crystallized region of a portion at which the amorphous silicon and the transparent regions overlap each other melts again and is crystallized as illustrated in FIG. 4C. In this manner, the laser beam is repeatedly scanned and radiated, such that the polycrystalline silicon of the crystallized region of the portion at which the amorphous silicon and the mask pattern transparent regions overlap each other melts and is solidified again. Consequently, crystallization is achieved.
FIG. 5 is a photograph of line muras or stripe effects displayed when polycrystalline silicon fabricated by SLS using the laser shot mixing technique is applied to a display.
As illustrated in FIG. 5, the shot mixing technique can reduce luminance non-uniformity but cannot completely remove the line muras or stripe effects. As such, the luminance non-uniformity cannot be perfectly corrected. In addition, the laser shot mixing technique uses a minimum 4 shot process to form one crystal, which requires an increase in processing time.
U.S. Pat. No. 6,322,625 discloses a method of depositing amorphous silicon on an entire substrate and then transforming the amorphous silicon on the entire substrate into polycrystalline silicon or crystallizing a selected region of the substrate using an SLS method.
In addition, in U.S. Pat. No. 6,177,301, large silicon grains are formed using an SLS method. When an active channel direction is parallel to a direction along which the grains are grown by the SLS method upon fabrication of a TFT for a liquid crystal display (LCD) device having a drive circuit and a pixel array, a barrier effect of a grain boundary with respect to the direction of an electric charge carrier is minimized, such that it is possible to obtain TFT characteristics similar to those of single crystalline silicon. However, according to these patents, when an active channel region is perpendicular to the direction of grain growth, there are numerous grain boundaries acting as a trap for charge carriers, and thus the TFT characteristics greatly deteriorate.
A TFT in a drive circuit and a TFT in a pixel cell region may be positioned generally at a right angle to each other when an active matrix display is fabricated. Here, in order to improve a uniformity of characteristics between the TFTs without greatly deteriorating the characteristics of each TFT, an active channel region is fabricated to extend along a direction forming an angle of between 30° and 60° with a crystal growth direction, such that it is possible to improve the uniformity of the device.
However, the above-described approach also uses the SLS method, and thus has a drawback wherein non-uniformity of grains of polycrystalline silicon may result due to non-uniformity of laser energy density.
An approach for configuring a laser beam pattern to have a triangular shape and inducing crystallization while horizontally moving the triangular beam pattern is disclosed in Korean Patent Laid-Open Publication No. 2002-0093194. This approach has a drawback because it can be nearly impossible to crystallize the entire substrate, with a non-crystallized region always remaining.